On Friday night, May 6, hundreds of bikes lean against the massive pillars holding up the Palace of Fine Arts' rotunda, a colorful array of plastic flowers and stereo speakers affixed to their baskets and trailers.

Their riders, flushed with endorphins after a four-mile cruise across town, are ignoring the winds whipping off the bay and dancing their asses off to a pumping sound-system that switches from bubblegum pop to John Lennon's "Imagine" and on to an electronica instrumental as more bikes arrive under the dome and the circle of dancers grows.

As a conga line forms, the dancers intermittently cheer "Bike party!" just as the cyclists have been declaring at the mini-parties held at every red light throughout this and the other monthly San Francisco Bike Party rides that started in January.

It's a celebratory moment for San Francisco bike culture, and a sign that it's branching off into new directions. While the venerable Critical Mass ride — which marks its 20th anniversary next year — seizes space on the roads, ignores red lights, and often sparks confrontations with motorists, Bike Party is a celebration that seeks to share space, avoid conflict, and just have fun.

Bike Party follows a set route on the first Friday night of every month, stopping two to three times along the way for dance parties. The basic idea is that participants should obey most traffic laws, stop at red lights, and try to avoid taking up more than one lane. And while Critical Mass is a local invention that was exported to cities around the world, Bike Party was imported from San Jose, where it started with the efforts of three 20-something roommates.

They were Nick Laskowski, who had helped to organize the by-then defunct San Jose Critical Mass ride; Amber Lamason, another organizer of San Jose social bike rides; and Lauryn McCarthy, an East Coast native new to San Jose who "just wanted to build community and meet people who liked to bike."

In a town hardly known for its great biking environment (despite its relative-to-San-Francisco flatness, bike riding on San Jose's freeway-like thoroughfares "can be really daunting to new riders," as one SJBP organizer put it) the three publicized their new Bike Party on Facebook, and 25 people showed up to the first ride in October 2007.

"We were stoked," McCarthy, who has since moved to San Francisco, recalled during an interview at a cafe on lower Divisadero Street. By June 2008, the monthly ride hit 120 riders, and one day a biker she didn't recognize invited McCarthy to join the ride. "I knew it had arrived."

S–M TO ANIMALS

These days, San Jose Bike Parties have monthly costume themes from S–M to animals, and can attract up to 3,500 riders. The events have gotten so large that organizers now wait to publish routes until 24 hours before the ride to cut down the numbers. Other chapters have sprung up (with the organizational help of San Jose core volunteers) in the East Bay and San Francisco.

How to explain Bike Party's instant popularity among Bay Area riders? It might be that its ethos appeals to a different sentiment than Critical Mass. While most Mass riders see that monthly ride as an opportunity to disrupt the automobile status quo, Bike Party is built around sharing the road.

It's been a welcome new addition to the scene for many longtime urban cycling advocates like Justin Fraser, who has long held a Critical Mass pre-party but who switched the event to precede the San Francisco Bike Party after having a great time at the maiden ride in January.

"I've been doing Critical Mass since the late '90s, and I usually go about 10 times over the course of the year, so I'm a regular. What I loved about Critical Mass is it's a great group bike ride." Fraser said.

But he and other regular riders often grew tired of the regular confrontations with angry motorists, the police presence, and the often circular routes through car-clogged downtown during rush hour that the leaderless Critical Mass ride would take.

"I love how it's a planned ride and you get out to other parts of the city, like the recent ride out to Candlestick Point," Fraser said. "Bike Party avoids lots of Critical Mass' conflicts by stopping at lights, getting out of downtown, and starting later."

Amandeep "Deep" Jawa, another longtime bike culture leader whose "Trikeasaurus," a three-wheeler tricked out with a booming sound system, is a familiar sight to many SF riders, has also warmly embraced Bike Party and volunteered his time to helping establish it here.

"I'm not sure whether it's an evolution or just something different," Jawa said, comparing Bike Party to Critical Mass. "I love both of them for different reasons. I don't think Bike Party is ever going to have that agit-prop element to it."

Indeed, Critical Mass was founded as an agitation-propaganda event to directly challenge the dominance of car culture, something Jawa says is still relevant and attractive to him. But Bike Party is a deliberate effort to broaden the appeal of group bike rides to larger audiences, which organizers say still has a political impact.

"Anytime you put bicycles on the road en masse, it's an inherently political act," says McCarthy. In an e-mail to the Guardian, the San Francisco Bike Party collective backed up her sentiment. "While SFBP doesn't specifically advocate for any politics or policies, by simply showing how many regular folks want to party on their bikes each month, we're showing that there is a need for a public space for people who ride bikes."

FIXIES AND FAMILIES

Contrary to much of the Bike Party's recent coverage by anti-Critical Mass media sources, which tend to represent it as the antithesis to the decades-old ride, the two events started with similar traffic policies and work to many of the same ends.

Like Mass, Bike Party practiced "corking" in its early stages in San Jose, assigning volunteers (or "birds," in the group's parlance) to post up in intersections to block cars for other riders as a safety precaution.

In 1997, Critical Mass experimented with stopping at red lights but soon eschewed the practice — it was considered too dangerous with the 5,000 to 8,000 people who were then riders. "It just meant a very long, slow-moving traffic jam," said Hugh D'Andrade, who has been involved with Critical Mass almost since shortly after its first ride in 1992 and created a website devoted to the San Francisco ride.

It wasn't until 2008 that Bike Party organizers decided to switch to the ride's current system of stopping at lights and sharing the road. "We thought it would be safer for our riders," said McCarthy. D'Andrade and friends rode in the San Jose Bike Party in early 2010, a ride he recalls was "so thoughtfully laid out, super celebratory, ethnically diverse."

That ethos seems to appeal to bikers at all levels of commitment and many walks of life. The San Jose rides now attract "mountain bikes, fixies, roadies — we have a cruiser bike gang that comes, even families," said McCarthy. San Francisco's ride, which officially kicked off Jan. 7 with a "happy birthday" theme, has yet to draw the thousands of people that Critical Mass or its San Jose counterpart do. But some bike activists we interviewed for this article felt like it was only a matter of time before it does.

D'Andrade now rides both events every month. He designed SF Bike Party's logo and now is a member of the group's planning collective, or "hub" as Bike Partiers refer to themselves. He said he feels the same vibe riding in both events.

But here's no doubt that the two rides were created at very different moments in San Francisco bike culture. "To ride through San Francisco in the early '90s was to take your life into your hands and be subject to harassment," he said. "Bicycling was not a mainstream transportation option."

Today, thanks to decades of Critical Mass Rides and concerted political advocacy work by people like Fraser and Jawa — both longtime board members of the San Francisco Bicycle Coalition — the city now offers an extensive bike lane network, near universal political support for bicycling, and packs of bicyclists on the road offering the safety of numbers.

"You wouldn't have this critical mass without the earnest approach of the Bike Coalition. But then, when all these people are out there cycling, it creates opportunities for things like the Bike Party," Jawa said. "There are just so many of us now, and so much joy around it, that people automatically get excited."

The sophistication of Bike Party's route planning and event management is another difference between the two rides. D'Andrade remembers the April 1 ride (themed "Robots and Cyborgs") when the group stopped at Children's Playground in Golden Gate Park, and caught in a moment of glee, swarmed the play structures en masse.

It was fun, but to D'Andrade, it just didn't feel quite as organic or spontaneous as the best moments of Critical Mass. As he said, "That kind of thing happens at Critical Mass, but here you know it was planned."

Luckily, there's no need to roll your wheels just one way. With the SF Bike Party on the first Friday of every month and Critical Mass on the last Friday, San Francisco bike culture has more than enough room for both events — and then some.